Where do the protons in oxidative phosphorylation come from?(The proton gradient is formed to make ATP through ATP synthase)I know in plants the proton gradient is made by splitting water (hydrogen specifically).Does it come from NADH & FADH2 in this case?

Originally I thought that they only provided the electrons for the ETC, but it not them, then where else can the electrons come from? This was unclear in my Barron's AP Bio book.